1998 Chevy Lumina clicks when trying to start

Happy new year.

My '98 Lumina was clicking when trying to start the last couple days. Radio time would reset to 1am, so I was completely losing power. Figured bad battery connection.

Dug into the battery, discon both cables from terminals. Clean with wire brush, re-attach. Get in car, have zero power anywhere. So then I discon the negative cable and check voltage from neg battery post to frame and it shows 11.5 volts. From positive post (that cable still connected) to frame, I have 1.4 volts. There's 12.9 volts direct across the two battery posts.

Assumed that I should have 0 volts from either terminal to the frame with neg cable removed from battery. Am I wrong? I do fine with the old 60's hotrods but I get lost quickly on the newer stuff, so my troubleshooting technique or assumptions might be totally off base.

I've inspected both main battery cables, they look fine. 0 ohm resistance end to end. Haven't touched anything else electrical in my efforts so far. I did detach that fusebox near the battery so I could move it out of the way to remove the windshield washer reservoir to get to the battery, but that's it.